Echo in a telecommunications network is a natural consequence of converting between a four-wire communication path and a two-wire communication path. Due to physical characteristics of human hearing mechanisms, echo can have a detrimental influence on voice communications. Consequently, a telecommunications network may include a device known as an echo canceler in a communication path to eliminate the echo in one direction of the path.
Digitally implemented echo cancelers are commonly employed in four-wire communication trunks to cancel echo in one direction for the channels carried by the trunk. Generally, such echo cancelers utilize an adaptive filter which monitors incoming signals to simulate the actual echo expected in outgoing signals. This simulated echo is subtracted from the outgoing signal to remove the actual echo, and the simulated echo is continuously adapted to keep the outgoing signal substantially echo-free with substantially zero loss in the outgoing signal's amplitude. As a result, voice communication greatly improves.
While echo canceling serves a valuable role in enhancing voice communication, it often harms the communication of digital data through a telecommunications network. Modems and other data communication devices which couple to a telecommunications network have markedly different characteristics from those exhibited by the human hearing mechanisms. Specifically, such data communication devices do not necessarily suffer a significant deterioration in communication when moderate amounts of echo are present. Additionally, such data communication devices do not exhibit the same half-duplex nature of communication as is characteristically exhibited by human voice communication. Thus, data communication, especially that which utilizes sophisticated techniques to transfer data at rates of 4800 bits/second or more, improves when a communication path omits echo canceling.
In order to accommodate both voice and data communication, echo cancelers employ circuits which control their operational status. Such circuits determine when to disable or bypass echo canceling and when to enable echo canceling. The telecommunications industry follows reasonable well-defined standards to control the disabling of echo canceling. In one commonly used technique, those data communication devices which require disabling of echo cancelers transmit a disabling signal through the telecommunication network as a preamble to data communication. CCITT recommendation G.164 defines this signal as being a simple 2100 Hz tone, while CCITT recommendation G.165 defines this signal as being a 2100 Hz tone with embedded phase reversals. Accordingly, echo canceler control circuits monitor the communication paths in which they are inserted for the presence of an appropriate disabling signal, and disable echo canceling when the disabling signal is detected.
On the other hand, no well-defined standards for enabling echo canceling exist. Rather, a variety of unacceptably expensive and unreliable techniques are used to enable echo canceling. For example, one technique requires the monitoring of signalling bits or common channel signalling (CCS) to detect when a communication channel changes from a busy to an idle state, or the like. Separate processors are required to monitor such signaling bits, and such signalling bits are often encrypted so that the monitoring of such signals requires the incorporation of undesirably expensive processing capabilities within the echo canceler.
Another technique calls for modification of central office switching facilities to transmit specifically defined codes, which are recognizable to an echo canceler, at the termination of calls. While such codes may be more easily decoded by an echo canceler than the signaling bits discussed above, great expense and complication is typically required in order to modify the operation of central office switching facilities.
Yet another technique enables echo canceling only when speech is recognized at the echo canceler. However, this technique again requires substantial and sophisticated processing power to implement circuits which reliably recognize speech as opposed to data communication. Moreover, this technique leads to poorer communication services because echo is present at the beginning of each voice conversation until the echo canceling becomes enabled.
Still another technique enables echo canceling after monitoring the energy being transmitted though the channel. When substantially no energy has been transmitted for an appropriate period of time, a call is assumed to have been concluded, and echo canceling is enabled. While most data communication does not have significant delays in the transmission of data, nothing insures that such delays cannot happen. Specifically, such delays often occur during half-duplex data transmissions when substantial delays are present in the long-haul side of the transmission path. Consequently, this technique is undesirably unreliable.